Various techniques are known for implementing active filters utilizing integrated circuits in bipolar or MOS technology. Such filters no longer require the use of expensive and cumbersome inductors, but do require the use of external capacitors, since high value capacitors and active elements are difficult to integrate on the same chip.
This problem has been partly solved by MOS technology switched-capacitor filters, in which the capacitors can be integrated.
Serious compatibility problems however arise when an MOS circuit must be inserted into a bipolar circuit, because of the different input impedances and breakdown voltages.
Unfortunately, switched-capacitor filters cannot be readily implemented in bipolar technology, since the available specific capacity is about half that obtainable in MOS technology, requiring large areas for the capacitive elements. The switches also can only be implemented with difficulty.
Active filters in bipolar technology can be implemented according to the method described in commonly owned Italian Patent Application No. 67809-A/85, (U.S. application Ser. No. 901,301 filed Aug. 27 1986,now U.S. Pat. No. 4,721,920 issued Jan. 26, 1988). It allows fully-integrated active filters to be made without external components, apart from two resistors for programming both the gain bandwidth product and dynamic range of the individual integrators.
The design is based upon conventional RLC (Resistor, Inductor, Capacitor) filters and arrives at an equivalent structure exclusively composed of operational amplifiers used as integrators, and of resistive dividers.
Such filters, known as "active R filters", have been first described by Michael A. Soderstrand in an article entitled: "Active R. Ladders: High-Frequency High-Order Low-Sensitivity Active R Filters Without External Capacitors" IEEE Transactions on Circuits and Systems, No. 12, December 1978.
An integrated circuit for the implementation of one or more filters of this kind comprises a number of resistors adapted to form resistive dividers, amplifying modules programmable as integrators or as input/output buffers, a bias circuit and access points for filter programming, inputs and outputs.
Moreover the transfer function of the desired filter should be able to be selected by programming a single metal mask according to the technology utilized in "structure array" integrated circuits.